The present invention relates to an inverter device, and more particularly to a method and apparatus for conducting regenerative braking for a motor load.
Pulse width modulation (PWM control system voltage type multi-inverter devices, such as that illustrated in FIG. 13 are well known as control devices for AC motors, such as variable speed control induction motors.
The device illustrated in FIG. 13 is an inverter, configured in a 3-phase star connection, which has 2 single-phase inverters 12 connected in series in one phase. The inverter drives a motor 13 from output terminals U, V, W. In this example, the inverter is configured with a transformer 11, which supplies a 3-phase power source that is insulated at each AC input of the single-phase inverters. The single-phase inverter 12 will be explained in detail with reference to FIG. 14. In FIG. 14, 3-phase AC current is converted to DC current by a 3-phase bridge rectifier 14, and is then smoothed by a smoothing condenser 15. The resulting DC current is then converted by a single-phase bridger inverter 16. This conversion is conducted by using PWM control to convert to an AC current with the desired voltage and frequency.
According to the configuration illustrated in FIG. 13, an output voltage three times greater than the voltage that a single-phase inverter outputs can be obtained, and a large capacity inverter can be configured. Shifting the PWM control timing of the two single-phase inverters, which configure one phase, doubles the PWM frequency that appears in the output, and an output close to a sine wave is obtained by the individual voltage steps becoming xc2xd when seen in terms of the phases overall.
In the aforementioned multi-form inverter, the PWM signal is generally conducted by shifting the carrier signal phase, as cited in pages 125 and 126 of xe2x80x9cSemiconductor Power Conversion Circuitsxe2x80x9d (Institute of Electrical Engineers of Japan publication, marketed by Ohm Co.), and as cited in U.S. Pat. No. 4,674,024 to Paice and U.S. Pat. No. 5,625,545 to Hammond.
An example of the output voltage waveform is indicated in FIG. 15. By switching alternately between the output voltages U1 and U2 of two single-phase inverters, a waveform generally closer to a sine wave can be obtained. Furthermore, shifting the phase of the secondary windings of the transformer 11 can reduce the high frequency at the input. In other words, the output is made smooth because 3-phase is output by all stages, and it becomes the so-called xe2x80x9c12 pulse configurationxe2x80x9d in which the phase of the transformer winding is shifted 30xc2x0, and the high frequency at the input can be reduced. FIG. 13 illustrates an example of two stages, but it is clear that 3 or more stages will result in obtaining even greater improvements.
Nonetheless, there are problems with these known systems. First, compared to a 3-phase inverter, the single-phase inverters have numerous circuit parts. Secondly, there is an output pulse double the frequency of the output at the DC part, and the use efficiency of the rectifier is poorer than the use efficiency of a 3-phase inverter because of the voltage pulse peak. In addition, the current ripple of the smoothing condenser is large. Furthermore, because the current ripple of this smoothing condenser has a big effect on the life expectancy of the condenser, it is necessary to guarantee sufficient design margins, and this becomes very expensive.
In addition, in order to process the load regeneration power, it is necessary either to consume DC power by resistance at all of the single-phase inverters, or to provide a special conversion circuit at the power source which becomes a problem as the size of the condenser increases. Further, there is also the problem that the fewer the single-phase inverter stages, the more the current input high frequency increases.
Consequently, in consideration of the aforementioned problems, the present invention has the purpose of offering an inverter device that can increase the inverter capacity and heighten the voltage by grouping a 3-phase inverter with single-phase inverters, and that, by reducing the number of parts, can improve reliability and economy, reduce the input high frequency, and realize at low cost a regeneration function.
According to one embodiment of the invention, a method for conducting regenerative braking for a motor load is disclosed wherein the load is driven by a plurality of power cells on each phase-line to the load, the plurality of power inverters comprising at least one single phase power inverter and at least one 3-phase inverter wherein the 3-phase inverter is connected across 3-phase lines. First, it is detected when the motor load is regenerating a motor load voltage. Then, each single phase inverter in each phase line is shorted out after detecting the motor load regeneration. The motor load voltage is then supplied to the 3-phase inverter Finally, the load motor voltage is lowered by conducting regenerative braking through the 3-phase inverter.
According to another embodiment of the invention, a power drive system is disclosed for driving a motor load which provides regenerative braking for the motor load. The system comprises a plurality of power inverters on each pulse line of the motor load, the plurality including at least one single phase inverter and at least one 3-phase inverter wherein the 3-phase inverter is connected across the different phase lines to the motor load. The system comprises mean for detecting when the motor load is regenerating a motor load voltage. In addition, the system comprises means for shorting out each single phase inverter in each phase line after detecting the motor load regeneration. The system comprises means for supplying said load voltage to a 3-phase inverter, and means for lowering said motor load voltage by conducting regenerative braking through the 3-phase inverters.